Abstract: In a process of manufacturing a semiconductor substrate having a SOI (silicon on insulator) structure, grooves are formed in a silicon layer reduced in thickness to several microns so that the silicon layer is separated into island-like regions corresponding to a chip size or device regions, and a stopper having a thickness corresponding to a desired final thickness of the silicon layer is formed in the grooves. The silicon layer is scanned with a piece of polishing cloth which has an area larger than that of each island-like region but sufficiently smaller than that of silicon layer and which is attached to a pressing surface of the polishing jig, thereby polishing the silicon layer until the stopper is exposed. The thickness of the silicon layer is measured at a position such that the thickness of a portion thereof is measured immediately before the same portion is polished.